Definition: flashes of light
More specific terms: Gaussian pulses, sech2-shaped pulses, parabolic pulses, solitons, quasi-soliton pulses, bandwidth-limited pulses, chirped pulses, double pulses, ultrashort pulses, laser pulses
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Author: Dr. Rüdiger Paschotta
Optical pulses are flashes of light. Particularly very short light pulses are often generated with lasers (laser pulses) and delivered in the form of laser beams, i.e., with a highly directional radiation.
Due to the enormously high optical frequencies, light pulses can be extremely short (ultrashort) when their optical bandwidth spans a significant fraction of the mean frequency. For example, a Gaussian pulse with a center frequency of 300 THz (corresponding to a wavelength of 1 μm) can easily have a bandwidth of 30 THz, and this already corresponds to a pulse duration of ≈ 15 fs if the pulse is transform-limited.
The shortest optical pulses generated directly in lasers (passively mode-locked titanium–sapphire lasers) have durations around 5 fs, corresponding only to a few optical cycles (few-cycle pulses). Pulse compression techniques applied to similar pulses reach pulse durations of very few femtoseconds, and high harmonic generation even allows the generation of attosecond pulses. On the other hand, many commercially important laser sources (particularly Q-switched lasers) generate nanosecond pulses (often with considerable pulse energies), which are considered as short but not ultrashort. Nanosecond pulses (from nanosecond lasers) also have many important applications, e.g. in laser material processing.
Depending on the required pulse duration, pulse energy, and pulse repetition rate, different methods of pulse generation, pulse compression and pulse characterization are used, overall covering extremely wide parameter regimes. See the corresponding articles for details, in particular the one on pulse characterization.
High Peak Powers and Intensities
Due to the short pulse durations and the potential for strong focusing, optical pulses can be used for generating extremely high optical intensities even with moderate pulse energies. For example, a 10-fs pulse with only 10 mJ energy has a peak power of the order of 1 TW = 1000 GW, corresponding to the combined power of roughly 1000 large nuclear power stations. This power may be easily focused to spots with a diameter of only a few micrometers. Therefore, amplified ultrashort pulses are very important for high-intensity physics, studying phenomena such as multi-photon ionization, high harmonic generation, or the generation of even shorter pulses with attosecond durations.
Characterization of Light Pulses
There are various methods for measuring the pulse duration achieved or for pulse characterization in other respects. Particularly for measuring the duration of ultrashort pulses, purely optical techniques are very important, since electronics are too slow for such purposes.
Single Shot or Repetitive Pulse Generation
Short laser pulses in the nanosecond pulse duration regime are often generated either in a single-shot regime (pulse on demand, with long and potentially irregular breaks between the pulses) or in a repetitive mode with a pulse repetition rate which is often in the kilohertz region. In contrast to that, ultrashort pulses (i.e., with durations in the picosecond or femtosecond region) are often generated as pulse trains with high repetition frequencies of many megahertz or even many gigahertz.
Bursts of Pulses
In some cases, laser sources do not generate a periodic sequence of pulses, but rather a periodic sequence of pulse bursts, where each burst consists of some number of short or ultrashort pulses. Within the larger a burst, one may have a high pulse repetition rate e.g. in the megahertz or gigahertz region, while the repetition rate of the bursts can be much lower, e.g. in the kilohertz region or even less.
For more details, see the article on burst mode lasers.
Pulse propagation in media has many interesting aspects. The peak of a pulse in a transparent medium propagates with the group velocity, not the phase velocity. Dispersion can cause temporal broadening (or sometimes compression) of pulses. For high peak intensities, optical nonlinearities can strongly affect the pulse propagation; often they lead to pulse broadening, but strong nonlinear compression is also possible.
Apart from experimental tests, details of pulse propagation can also be investigated with various kinds of numerical simulation. In some cases, for example for pulse propagation in single-mode fibers or free-space propagation with a fixed Gaussian beam profile, one can neglect the transverse spatial dimensions and consider only the complex amplitude versus time or frequency at each location. More sophisticated numerical models are required for investigating the full spatio–temporal pulse evolution.
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|||R. Paschotta, Field Guide to Laser Pulse Generation, SPIE Press, Bellingham, WA (2007)|
|||R. Paschotta, tutorial “Modeling of Pulse Amplification”|
See also: pulse trains, ultrashort pulses, double pulses, pulse duration, pulse energy, pulse repetition rate, carrier–envelope offset, spectral phase, pulse generation, pulse characterization, pulse propagation modeling, pulsed lasers, burst mode lasers, ultrafast lasers, mode-locked lasers, Q-switched lasers
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